This invention relates to a cold trap, and more particularly, to a structure of a mesh section suitable for being used for a cold trap collecting impurities in liquid metal.
A mesh section incorporated in a conventional cold trap has a structure in which a mesh screen woven of fine metallic wire in a stockinette stitch state is corrugated and wound around a tubular member as disclosed in U.S. Pat. No. 3,831,912. In such mesh section, the surface of the mesh screen is located in a cold trap in such a manner that it is parallel to the flow of liquid sodium. Also, a mesh screen plain-woven of fine metallic wire is well known.
There is a cold trap such as described in Japanese patent application Laid-Open No. 56-166344 (166344/1981) (corresponding to U.S. Pat. No. 4,291,865) in which, in mesh bands in the cold trap, a mesh band of small mesh size is located at the inner side of the mesh bands defining an outlet for liquid and a mesh band of large mesh size is located at the outer side of the mesh bands which defines an inlet for liquid to be supplied, thereby flowing liquid metal radially from the outer side of the mesh bands to the inner side to collect impurities contained in the liquid metal by the mesh bands.
A cold trap shown in FIG. 1 of Japanese patent application Laid-Open No. 62-33729 (33729/1987) is also well known, in which mesh bodies are provided at multiple stages, and when the lower-stage mesh body is choked, liquid is flowed into the middle-stage mesh body to remove impurities in the liquid by the middle-stage mesh body, and further when the middle-stage mesh body is also choked, the liquid is flowed into the upper-stage mesh body to remove the impurities in the liquid by the upper-stage mesh body.
In all well-known instances above-mentioned, when differential pressure is created between an inflow port and outflow port of liquid sodium in the mesh section, displacement or break of the mesh screen often occurs. The conventional cold trap is ordinally designed so that the filling density of the mesh section is about 5-15% with respect to the capacity of the cold trap, and the effective rate of collecting impurities is about 10-30%. Such a small collecting effective rate is considered to resulted from the following causes.
That is, the impurities in the liquid sodium may be omnipresently collected through the mesh section at the inflow and outflow sides for the liquid sodium of the cold trap and a large amount of the impurities will be obtained at the beginning of operation of the cold trap. For this reason, the fluid resistance of the liquid sodium at the mesh section becomes large, which results in an increase of the differential pressure between the inflow and outflow ports for the liquid sodium of the mesh section. Accordingly, the mesh section is crushed in the direction of the flow of the liquid sodium, and a portion of the mesh section is closedly in contact with the adjacent portion thereof so as to make the flow of Na uneven. The portion of the mesh section is plastically deformed by the break thereof so that the primary efficiency cannot be maintained. The collecting efficiency of impurities is thus decreased. Also, it is difficult to regenerate the cold trap and the primary efficiency cannot be recovered.